Buttons are grouped in the following pattern
Y Y - eject, buttons in parallell
Y Y - phone, buttons in parallell
X X - previous and next, X3 left and X3 right below
X Y - <> and clock, X4 left and Y4 right below.
X X - tone and select, X5 left and X5 right below
X X - menu, X6 left and X6 right below. Not in parallell

Reason for this grouping I believe is because some buttons work when ignition is off and some don't.

From the top the 20 connections are
1 NC
2 Common for buttons X
3 LED GND
4 LED GND
5 X6 right
6 X5 right
7 NC
8 X3 right
9 Buttons row 2
10 Yellow LED
11 Red LED
12 Green LED
13 Buttons row 1
14 X3 left
15 X4 left
16 X5 left
17 X6 left
18 LED + for all backlight LED's
19 Common for buttons Y
20 Y4 right